Rhythmic vibration above 70 mph

[xavier296]

On my 67 galaxie, the car has a rhythmic vibration above 70 mph since I did the full redo including the new engine. It feels like a wave of vibration, with a period of 5 or so second, gets stronger, then weaker, stronger, etc. Tire man says the wheel balance is good. Drives me nuts feeling the whole car shake, especially when the wife dismisses it as "the problem with old cars!!!" Any previous experiences that might lead me to a solution?

Thanks, Aaron

[andrewb70]

Aaron,

What kind of engine and trans mount are you running?

Andrew

[xavier296]

Stock transmission crossmember and rubber mount. Home fabricated frame mounts. Ford 4.6L engine mounts.

[andrewb70]

Stock transmission crossmember and rubber mount. Home fabricated frame mounts. Ford 4.6L engine mounts.

Have you measured your front and rear u-joint operating angles?

Andrew

[jlcustomz]

Had a similar problem for years which I hope to cure with adjustable upper control arms & an angle gauge when I get a chance.
Spicer has a demonstration video online showing the effect of driveline angles.
https://www.youtube.com/watch?v=gmV4qwLfOMY

This exaggerated demonstration gets the point across
Haven't tried it yet either, but Tremec has a phone app to measure driveline angles.

[Peter Mc Mahon]

Shift into neutral and see what happens

[TheJDMan]

That high speed harmonic vibration is most likely in the drive shaft. As mentioned above you need to measure the U-joint operating angles. There are also a number of threads on this subject.

https://www.pro-touring.com/threads/104559-Optimum-driveline-angles-for-vibration-free-high-speed-operation?highlight=vibration

[xavier296]

Everything is adjustable on the 9 inch with adjustable arms. I thought I found the problem 6 months ago when I measured a 4 degree driveshaft angle at the yoke, but changing it to 1.5 degrees made no difference. Not ruling out the driveshaft, just haven't figured it out yet.

[andrewb70]

Everything is adjustable on the 9 inch with adjustable arms. I thought I found the problem 6 months ago when I measured a 4 degree driveshaft angle at the yoke, but changing it to 1.5 degrees made no difference. Not ruling out the driveshaft, just haven't figured it out yet.

The operating angles are the angles that the transmission output shaft and the pinion yoke make to the driveshaft. So with the cars weight on the suspension, you need to measure the angle of the output shaft, the angle of the driveshaft, and the pinion shaft angle. Pay close attention to which direction everything is pointing.

My guess is that your front operating angle is too large, which is common for cars that are lowered.

Andrew

[GrabberGT]

I chased this problem for a while on mine. Turned out that a large contributor to the issue was the flat spotted tires. Your tire guy prob checked for that but for anyone else reading this...

[Cyclone03]

Mid 60's Fords have the engine angled 4.5deg trans down. Level the car at the rocker panels. If it was carbed remove the carb and the carb pad should be level. On the mod motor put your angle gauge on the crank pulley. The rear axle should be 4.5deg pinion up with the axle loaded and same zero angle at rockers.

All that said do you have an OD trans? What is your rear gear?
On my Mustang I went to a 3" dia ( forgot the wall thickness) drive shaft to help quite the waa waa's . Don't over look the rear gear set up as well, and some gear brands can be noisy too.
My .62 OD and 3.70 rear gear gets the waa waa's at about 110mph now,80mph before the drive shaft change.

[CopperLark]

I agree with Cyclone. A bad rear differential can cause a rhythmic vibration like you describe. Had to rebuilt two of them for friends to get rid of it.
Good luck.